Synthesis of prostaglandin (PG) F2Alpha by the ovary is known to be necessary for ovulation to proceed in an orderly fashion. Our studies have demonstrated that FSH stimulated the synthesis of prostaglandins (PG) E and F2Alpha by rat ovarian granulosa cells in a dose-related fashion augmented by exogenous hCG. PRL also exerted marked effects on PG synthesis. Physiologic amounts of PRL increased the rates of PGE and PGF2Alpha synthesis whereas injections of supraphysiologic amounts of PRL decreased the rates of synthesis. The stimulation by PRL over such a narrow concentration range is in agreement with in vitro studies on ovarian steroidogenesis. The role of PG in ovarian function was also studied using a newly devised assay for prostaglandin receptors which allowed further study of the role of PG in ovarian sterodogenesis and ovulation. Suppression of endogenous PRL secretion by bromoergocryptine or hypophysectomy caused an increase in the number of PG receptors in ovarian membranes, an effect reversed by replacement of PRL in physiologic amounts. Supraphysiologic concentrations of prolactin resulted in a marked decrease in the number of PG receptors. The observations suggest that prolactin exerts its effects through the PG cascade and explains, in part, the mechanism by which infertility occurs in hyperprolactinemic women.